This clinical trial, published Tuesday in the Journal of the American Medical Association, randomly assigned 222 overweight, inactive kids to one of three activity groups for 13 weeks.

The first two groups underwent 20 or 40 minutes of daily aerobic exercise in an after-school program. The third group went about their usual routines.

The researchers found that 20 minutes of exercise for just a few months was helpful compared to no exercise at all, said the lead author of the study, Catherine L. Davis, a professor of pediatrics at the Medical College of Georgia.

Those kids who were active for just 20 minutes a day were more fit, had less body fat, and had better markers for diabetes risk when compared to kids who weren't active.

The authors also found that health effects of exercise were the same for boys and girls and for different races.

Predictably enough, the study also found that more exercise is better. Kids who exercised longer had even less body fat and better values for markers of diabetes risk.

What was surprising is that these benefits occurred even without changing what kids ate.

"This study helps to isolate the benefit of exercise in cutting down on diabetes risk and obesity in kids," said study co-author, Dr. B. Adam Dennis, an endocrinology fellow at Georgia Health Sciences University in Augusta, Ga.

More research is needed to see if these results last, caution the researchers.

Dr. David L. Katz, editor-in-chief of the journal Childhood Obesity, said the study shows "even a little bit of physical activity might be the difference between a child developing diabetes or not." Katz was not involved with the study.

These findings could help policymakers when it comes to redesigning physical activity opportunities at the local, national, and global level.

"I hope these findings will provide an impetus for changes in communities around the U.S. and the rest of the world that will focus attention on children's health," Davis said. "This can be done by providing welcoming, safe physical activity programs for children of all skill levels."

This school-based study suggests that the "physical" may need to be put back into "education." Davis suggested that schools are a natural focus for exercise programs. While the creation of after-school exercise programs might be necessary to ensure 40 minutes of daily exercise, she said, 20 minutes a day of exercise can be achieved during regular school activities, like recess and gym class. She added that exercise is not just useful in cutting back diabetes risk and obesity, but it is also good for kids' brains. She cited a related study that showed that exercise improved cognition and math skills in kids.

Short exercise breaks in the classroom can easily be included during the school day, said Katz, who is also the co-creator of an in-class exercise program that offers online, free resources for educators.

This study had an unusually low drop-out rate -- perhaps because the activities were simple, fun, and appealing to kids of all athletic abilities, Davis said. The researchers also used low-cost prizes to reward kids for effort rather than performance.

"We purposely de-emphasized performance," said Davis, adding that it didn't really matter if they got the ball in the hoop as long as they achieved their target heart rate.

Their findings have important implications for designing school exercise programs in the future, she said, because many other programs tend to weed out less physically skilled kids and focus on a smaller group of more athletic kids.

One-third of American elementary school kids are overweight or obese, according to 2008 statistics from the U.S. Centers for Disease Control and Prevention. Type 2 diabetes is one of the most deadly and devastating consequences of obesity -- hence the importance of finding out how much exercise kids should get in order to prevent it.

Katz said the study shows that when it comes to how much exercise is helpful in cutting diabetes risk, "some is better none; more is better than some."